The present invention relates generally to control systems for electric motors and, more particularly, to a controller for regulating the magnitude and direction of the torque of a brushless DC motor in response to a low-level torque command signal.
Brushless DC motors are widely used in servocontrol applications. Typically, the motors used in these applications are controlled by commutating the current supplied to the stationary windings. Rotor position sensors are usually included as an integral part of the motor and provide the controller with signals that indicate the position of the rotor magnets relative to the stator windings. To achieve high efficiency, control of the motor current is accomplished by pulse width modulating solid state switches, usually power-switching transistors. This current regulation technique is, however, difficult to control since the relationship between motor current and pulse width is complex and highly nonlinear. Current control is further complicated by the wide range of motor velocity and back EMF.
If the motor and controller are to form the basis of a servocontrol system, it is highly desirable that the nonlinearity attributable to the switching mode nature of the current regulation technique be eliminated. Approaches aimed at this objective have typically enclosed the pulse width modulator and transistor switches within a feedback loop, with a current-sensing circuit being added to provide a feedback signal for this current control loop.
To implement a position servo, it is necessary to further provide both position and velocity feedback loops to obtain suitable performance. For high-quality applications, the performance of the position and velocity feedback circuits are critical to the ultimate performance of the complete control system. This requires that the controller maintain a linear relationship between the output torque and the commanded torque in all four of the so-called quadrants, or modes, of motor operation. These quadrants correspond to the four possible combinations of speed and torque produced by the motor when operating either as a motor or a generator and rotating in either a clockwise or counterclockwise direction. For high performance, the control system must respond quickly to torque commands and maintain a linear torque relationship independent of the "external" effects of supply voltage and motor speed. It is particularly important that the controller be capable of maintaining linear control while smoothly effecting a transition from one quadrant of operation to another.
In servocontrol applications, there is often a current command signal having positive and negative values corresponding to the desired direction of motor torque. For example, positive and negative command signals may correspond to desired clockwise and counterclockwise torque. For smooth, efficient control, it is necessary to provide the commanded clockwise or counterclockwise torque quickly and without regard to whether the motor is rotating clockwise or counterclockwise when the command is given, i.e., the controlled current output should be independent of the "direction" or polarity of the motor velocity. Where the nature of the command requires the controller to change operation from motoring or regenerating in one direction to the opposite mode in the same direction (e.g., from motoring in a clockwise direction to regenerating in a clockwise direction), a fairly smooth transition can be made by responding to the sudden reversal in the polarity of the loop error signal. When, however, the nature of the desired torque is such as to require a change from a regenerating mode in one direction to a motoring mode in the other direction (e.g., from regenerating in a clockwise direction to motoring in a counterclockwise direction), there is a problem created by the required change in the direction of the motor velocity. In particular, there is the problem of sensing when it is necessary to change the excitation of the motor in order to effect a smooth transition to operating in the reverse direction. This change in excitation must be made before the motor actually reverses its direction of rotation in order to assure a smooth transition and produce the desired torque.
To ensure reliability of the power-switching transistors, it is desirable to provide a safety circuit that will turn off all of the transistors (regardless of the demands of the current control loop) if transient conditions should momentarily allow the current to exceed the operating limit of the transistors. When the out-of-limit current condition no longer exists, it is necessary to signal the controller to assume operation and provide power to the switching transistors. Prior approaches to providing this resumption of control have typically employed a timer that cuts off power to the transistors in response to an out-of-limit signal and signals the controller to resume operation after expiration of a preset time. Since the duration of the over-current condition does not necessarily coincide with the preset interval during which power is off, undesirable limit cycles are introduced by these protection circuits.
The feedback current against which the command signal is measured to derive the error signal is typically obtained through the employment of complex, and hence costly, magnetically coupled sensors.
The present invention provides a motor controller that regulates the current applied to the windings of the motor in a manner such that linear control of the motor output torque is consistently provided in all four quadrants of motor operation. An important aspect of the invention is the provision of a circuit that assures smooth operation by determining when it is necessary to change the motor excitation (i.e., the excitation sequence of the windings) in order to produce the desired motor torque. In accordance with this aspect of the invention, smooth transitions from one mode of operation to another (particularly from a regenerating mode in one direction to a motoring mode in the other direction) are effected by a circuit that monitors the pertinent parameter, the current error signal, rather than an approximation of what is required.
A further aspect of the invention is the provision of a safety shutdown circuit that inhibits the delivery of power to the switching devices only for as long as is required to allow the current to fall back into a safe, controllable range.